Engine braking, including compression release brakes, are typically used to slow down momentum of a vehicle having an internal combustion engine after the driver has released, or is no longer engaging, the accelerator, such as releasing the operator's foot from the accelerator pedal. Engine braking may provide a way for slowing the movement of a vehicle that assists, or in certain circumstances operates in lieu of, traditional service brakes, such as friction brakes or magnetic brakes.
Traditionally, an engine brake, such as, for example, two or four-cycle Jake brake, develops its braking horse power by converting a moving vehicles forward momentum into mechanical work. More specifically, when the driver is not engaging the vehicle's accelerator, and the supply of fuel to the cylinders is shut off, the rolling of the tries or wheels of the vehicle causes the continued movement of the vehicle's drivetrain. Such continued movement of the drivetrain forces the crankshaft to continue to rotate, which is translated into the continued displacement of the pistons within the cylinders. Accordingly, as such displacement of the pistons includes the pistons undergoing a compression stroke, air within the cylinder continues to be is compressed as the pistons are displaced toward a top dead center position in the cylinder. With compression release braking, the pressure of such compressed air in the cylinder provides a force that generally opposes this displacement of the piston. Moreover, the opposing force, or engine braking horsepower, of the compressed air during such a compression stroke may slow the movement of the pistons, and thereby assist in slowing the momentum of the vehicle.
Further, when the piston approaches, reaches, or passes an upper position in the cylinder, such as the top dead center position, the compressed air may be released from the cylinder, such as through an exhaust valve in the cylinder head. An intake valve may then be re-opened so that intake air that is to be pressurized by the subsequent compression stroke enters into the cylinder during an intake stroke. According to certain applications, this process may continue to be repeated until the engine speed and/or vehicle is reduced to a desired level, such as, for example, the crankshaft being reduced to a range of revolutions per minute (rpm) (e.g. 1000 rpm).
One of the parameters that influence the amount of power developed by a compression release brake is the amount of compressed air delivered to the cylinder by the turbocharger during the intake stroke. For compression release brakes, engine control systems typically control the turbine side of the turbocharger to limit boost performance of the compressors that compress the air that enters the cylinder during an intake stroke. For example, a turbine(s) often uses exhaust gas to generate power that is used by compressor(s) to increase the pressure, and thus mass, of air that is supplied to the cylinder during an intake stroke. Systems may therefore limit or reduce the quantity of exhaust gas used by the turbine to reduce the amount of compressed air delivered to the power cylinder by the compressor(s) during the intake stroke. One example of decreasing the quantity of exhaust gas used to operate the turbine is by changing the position of the vane located at the inlet of the turbine of a variable-geometry turbocharger (VGT). Such alteration of the vane position may result in a reduction in exhaust energy available to, and power generated by, the turbine, and thereby decreases the amount of power available to the compressor to compress air. Such a reduction in compressor power may result in a reduction in the quantity of compressed air that is delivered to the intake valve and associated engine cylinder during the intake stroke.
However, such adjustments of the turbine vane may be, at least during certain operating conditions, insufficient to decrease the quantity of air being compressed by the compressor. For example, at high engine speeds, even with a change in the vane position, the turbine may still generate sufficient power for the compressor to compress a larger quantity of air than is desired for the engine brake. As a consequence, a larger than desired quantity of compressed air may enter into the engine cylinder during the intake stroke of an engine braking event. Moreover, such an overload of compressed air in the cylinder may result in an over boost in the force opposing the displacement of the piston during the compression stroke(s), thereby causing undue stresses on engine components, including an overload of the camshaft brake lobe. Further, such over boost may result in the production of unacceptable camshaft hertz stresses when the exhaust valve is open as the piston attains a top dead cylinder position in the cylinder.